1. Field of the Invention
The field of art to which this invention pertains is tenter clips.
More specifically, this invention involves a tenter clip which may be used to simultaneously stretch a web of film biaxially in the machine and cross-machine directions in a tenter frame. The clip consists of an anvil and a pivotable toggle arm with a curved gripping surface. When the toggle arm is pivoted into its operative position, the web is effectively gripped between this surface and a gripping surface of the anvil. The portions of the anvil surface adjacent this gripping surface are tapered. The surface configurations of the anvil and toggle arm are such as to permit the web to stretch out from beneath the gripping surfaces during the biaxial stretching operation.
In another more comprehensive embodiment, this invention is a tenter frame for stretching film using a plurality of these clips. The gripping surfaces of such clips have a longitudinal dimension in the machine direction which is less than the distance between the centers of the spaced clips, in their startup positions prior to stretching, to permit effective biaxial stretching to take place. The ratio of the longitudinal dimension of the gripping surfaces of the clips to the spacing distance between the center of the clips is between 0.1 and 0.6 and, preferably, between 0.2 and 0.35.
2. Description of the Related Art
It is well known in the art to use tenter clips having a shaped gripping surface for gripping a web of film during a biaxial stretching operation in a tenter frame.
Typical examples of such clips are seen in U.S. Pat. Nos. 3,305,889 to Lewis et al; 3,150,433 to Kampf; 3,391,421 to D'Onofrio; and 3,890,421 to Habozit. In these patents the gripping surface of the pivotable clamping member has a curved configuration; however, the flat gripping surface of the anvil limits the ability of the film to stretch smoothly biaxially in the machine and cross-machine directions under such gripping surfaces during stretching.
In the clip of the present invention the gripping surface of the anvil is comparatively narrow in width and has tapered surfaces at each of its edges. These features have been found not only to improve the stretching step but as the web stretches under the gripping surfaces, and decreases in thickness, the curved surface of the toggle arm and tapered surface of the anvil adjacent the edge of the web form a gripping means or region for gripping the bead which forms at the web edge, during stretching. This bead as it wedges in this substantially V-shaped gripping region enables the clip to carry most of the stretching forces and helps prevent undue stress concentrations and tears in the film during stretching.
It further has been found important to limit the longitudinal dimension of the gripping surface of each clip in the machine direction so that it is less than the distance between the centers of the spaced clips, at startup and prior to and during the stretching operation. By limiting the ratio of gripping surface length to the spacing distance between clips, more effective stretching between such clips is accomplished, particularly in the machine direction.
The present invention, using tenter clips as described above, accordingly provides an effective means for improving the stretching of a thermoplastic film biaxially in a tenter frame over that seen in the prior art. The web stretches more smoothly with less stress being placed on the web, and with fewer tears.